Instrumentation amplifiers are extensively used in analog systems to amplify low level signals in the presence of high common mode noise. As an example, instrumentation amplifiers are typically employed as input amplifiers of signal processing systems. Moreover, instrumentation amplifiers can amplify a differential input signal with a high rejection of common mode input voltage. Typically, instrumentation amplifiers have high input impedance, low input current, and good linearity.
The common mode input range of the amplifier is limited by the following constraint: The input stage of the two amplifiers constituting the instrumentation amplifier must be properly biased. To overcome this constraint, conventional systems employ two differential input pairs of transistors within the instrumentation amplifier, one based on n-channel metal-oxide-semiconductor (NMOS) transistors, and one based on p-channel metal-oxide-semiconductor (PMOS) transistors. The PMOS based differential input transistor pair works accurately for a low common mode input voltage range but not for a common mode input voltage range that is close to the positive supply voltage (Vdd). On the other hand, the NMOS based differential transistor pair works accurately for a high common mode input voltage range, but not when the common mode input voltage range is close to the negative supply rail voltage (Vss). Thus, by combining PMOS and NMOS based differential transistor pairs, the input stages of the conventional amplifiers constituting the differential amplifier can have a rail to rail input range. However, the gain and hence the performance of the conventional amplifier falls down rapidly when the output voltage approaches the positive or negative supply rail (e.g., within a few tens of millivolts).